Due date calculator: three ways to find your due date
Your due date anchors your whole pregnancy — every scan, screening, and milestone is scheduled around it. Here are the three standard ways to calculate it, and what to do once you know it.
Method 1: from your last menstrual period
The classic calculation (Naegele's rule): take the first day of your last period and add 280 days — exactly 40 weeks. This is the method most healthcare providers use by default, and it works well if your cycles are fairly regular.
Method 2: from your conception date
If you know when conception happened — for example through ovulation tracking or fertility treatment — add 38 weeks (266 days). The two-week difference versus the LMP method accounts for the time between your period and ovulation.
Method 3: from an ultrasound estimate
An early dating scan measures your baby directly and is often the most accurate estimate of all. If your provider gives you a due date from an ultrasound, use that one — it overrides the calendar math.
What a due date really means
Only about 1 in 20 babies arrives on the due date itself. Think of it as the middle of a window: anywhere from week 37 to week 42 is considered normal timing. The due date's real job is to keep your care on schedule — and to give you a number to count down to.
Calculate it once in Lunera, then never think about it again
Lunera supports all three calculation methods — last menstrual period, conception date, or a known due date. Pick whichever you have during a 30-second setup, and the app handles the rest:
- A live due date countdown ("136 days to go") on your home screen and widgets
- Your progress through all 40 weeks as a percentage
- Automatic trimester detection as the weeks pass
- Got a new estimate after a scan? Change the date in settings and everything recalculates
Quick answers
Can my due date change?
Yes — it's common for a first-trimester ultrasound to shift the estimate by a few days. Your provider will tell you if the official date changes; just update it in your tracker.
How likely is birth on the due date?
Around 5% of babies are born on their exact due date. Most arrive within a two-week window either side, which is why weeks 37–42 are considered term.
Track your pregnancy with Lunera
Week-by-week baby development, due date countdown, widgets, and a one-tap contraction timer — free, private, and subscription-free on iPhone.
This guide is for informational and educational purposes only and is not medical advice. Lunera is not a medical device. Always consult your doctor, midwife, or healthcare provider with any questions about your pregnancy or labor.